1. Field of the Invention
This invention relates to diffused metal oxide semiconductor (DMOS) transistors and more particularly relates to lateral DMOS structures suitable for high-power operation.
2. Description of the Prior Art
DMOS power transistors are well-known in the art, but most prior art devices have their drain electrodes connected to the device substrate. Therefore, when the device is mounted in a power transistor housing an undesirably high drain capacitance is produced In prior art lateral DMOS power transistors a low drain capacitance is achieved, but the drain to source breakdown voltage is determined by the device surface conditions. Therefore, when breakdown does occur, and a rush of drain current develops, the result is a surface damaged structure which is no longer useful.
The processes used to create the structures disclosed herein can be achieved using the conventional elements found in planar PN-junction isolated monolithic integrated circuits. For example, the processes and structures disclosed in the book ANALOG INTEGRATED CIRCUIT DESIGN, by Alan B. Grebene (Litton Educational Publishing, Inc. 1972) can be employed. The teaching in this book is incorporated herein by reference.
FIG. 1 shows a conventional power DMOS transistor configuration. While a single device is shown, a power transistor will involve a suitably large number of such devices connected in parallel. The number being determined by the current carrying requirement. The device is constructed upon an N+ wafer 10. An epitaxial layer 11 is grown on substrate 10 and is doped N-type to a level desired for the DMOS transistor drains. A P-type body 12 is diffused into the epitaxial layer to create a junction therewith. Body 12 is shown as a pair of diffusions but it can actually be a variety of forms. Its shape at the silicon surface determines the outer confines of the DMOS transistor. The shape can be round, oval, rectangular, hexagonal, interdigitated, or any other form as determined by the designer.
A source 13 is diffused into the body 12 region so as to extend part way therethrough. It is designed to be spaced a controlled distance away from the inner confines of body 12. A thin gate oxide 14 is formed at the silicon surface to span that portion of body 12 that exists adjacent to source 13. A gate electrode 15, preferably composed of conductive polycrystalline silicon (polysilicon) extends over the gate oxide 14 so as to span the space between the source regions as shown.
A field or thick oxide 16 is typically formed over the silicon surface at the perimeter of the immediate device area. A layer of oxide 17 is usually formed over gate 15 so that the polysilicon electrode is fully encapsulated in insulation. A cut is made in the surface oxide layers so as to expose portions of both the body 12 and source 13. Metallization 18 is formed on the device and etched back as shown so as to contact both the source and body. This metal forms the device source as shown schematically at 18'. The gate contact is shown schematically at 15' and the drain contact to the N+ substrate is shown at 10'.
In operation the PN junction formed between body region 12 and drain region 11 is back-biased by the applied voltage The conduction across such a reverse-biased junction is due to leakage which is quite small. When the gate 15 is positively biased its electric field can invert the conductivity of the adjacent body and create an N-type channel (not shown) that extends along the silicon surface from source 13 to drain 11. This channel can conduct and, if the gate is biased sufficiently, the conduction can be substantial. Electrons from source 13 can flow across the channel in body 12 and into drain 11 where they pass our via substrate 10 to electrode 10'.
The structure of FIG. 1 produces an excellent DMOS transistor, but several drawbacks are evident. Since the gate spans the inner portions of body 12 it must also span a significant portion of drain 11. This will lead to a substantial gate-to-drain overlap capacitance which is parasitic and detrimental to high frequency performance. It will be noted that substrate 10 forms the drain conduction path. In the case of a power transistor structure, where the DMOS transistor is housed in a power package, the drain electrode will ordinarily be connected to a metal housing. This will be the metal case for a TO-3 housing or the metal tab of a TO-202 or TO-220 housing. Such structures involve substantial additional stray drain capacitance which must be charged and discharged if the drain voltage is to be modulated.
FIG. 2 illustrates a lateral DMOS power transistor structure which avoids most of the drain capacitance problem. In this device a P-type silicon substrate 21 is employed in fabrication. The substrate resistivity is chosen to provide the desired DMOS body performance. A P+ body contact is diffused into the silicon surface at 22. This provides a topside ohmic contact to the substrate. An N+ source 23 is also diffused into the silicon. As shown, the source and body regions abut each other and are shorted together by a metal contact 24. Body contact and source regions 22 and 23 are linear structures that extend along the silicon surface and their extent defines the DMOS transistor shape. Metal contact 24 also extends along the silicon surface to the same extent.
A more heavily doped body region 25 is incorporated into the structure. As shown by dashed outline, this body region extends into the P-type substrate around source 23. Region 25 is achieved using an impurity diffusion that is extended beyond the confines of source region 23.
A drain electrode is diffused into the silicon substrate at 26. This N-type drain region extends over the silicon surface parallel to source 23. It forms a PN junction with the P-substrate and an N+ contact region 27 makes a low resistance ohmic connection to region 26. It is to be understood that the right-hand portion of FIG. 2 represents the center of the drain diffusion 26. In the actual structure a mirror image of that shown exists to the right. A second P+ region along with a second N+ source are spaced away from and facing the other edge of drain diffusion 26. From a process standpoint, P+ region 22 can be produced using an IC boron "isolation-like" diffusion and region 25 by an "NPN transistor base-like" diffusion. N+ regions 23 and 27 can be "an NPN transistor emitter-like" diffusion. Drain region 26 is typically a phosphorous diffusion that is driven in during the drive-in of P+ region 22. It is preferred that ion implantation be employed to deposit the various impurities because such a process is easily masked and can produce precise quantities of dopant. Thus, it can be seen that the topology of the device is determined by the surface shape of P+ region 22 and its mirror image.
Gate 28 is preferably composed of conductive polysilicon and is located over a thin gate oxide 29. This gate is employed in the fabrication process to self-align the formation of source region 23. This self alignment will locate the right hand portion of source 23 with respect to the left hand portion of gate 28. The other side of the gate, the right-hand portion, is in self register with N--region 33 which is a lightly doped diffusion that extends to drain 26. As shown in FIG. 2, gate 28 is also overcoated with an insulating oxide 30 which can either be grown out of the gate polysilicon or deposited thereon. The right-hand edge of gate 28 is made to lie well inside the edge of drain diffusion 26 as shown. The dimension and doping concentration of N--region 33 determines the breakdown voltage of the device. A metal contact 31 makes an ohmic connection to N+ contact region 21 in the conventional manner and the metal is caused to extend up over the planar oxide 32. This metal extension is terminated before the edge of gate 28. However, if desired, metal 24 can be extended up and partially over gate 28.
It can be seen that source region 23 is separated from drain extension 33 by a portion of P-type silicon substrate that acts as the DMOS transistor body. When gate 28 is biased sufficiently positive, the adjacent P-type body material becomes inverted and a channel will exist between source 23 and drain extension 33. If body region 25 is more heavily doped P-type than substrate 21, the gate will have to be biased more positively to create a channel and the DMOS threshold is higher. However, if desired, body region can be compensated by diffusing N-type impurities into the silicon at 25 and the DMOS threshold will be lowered to that level determined by the substrate 21 doping. Clearly the threshold can be controlled independently from the other device parameters.
It will be noted that the drain capacitance is only that produced by the drain PN junction. Furthermore, the gate to drain capacitance is minimized. So too is the gate to source capacitance. Finally, since the substrate is connected to the housing, the source/body contact is connected to the metal tab or housing in a power structure.
The major problem with the structure of FIG. 2 is that drain avalanche breakdown voltage is initiated by the silicon surface where region 33 forms a junction with substrate 21. This breakdown is therefore a function of the nature of the silicon-oxide interface which is difficult to control.